(a) Field of the Invention
This invention relates generally to the art of amperometric analysis and to devices of the type used for quantitative electrochemical determination of the concentration of an electroactive species of interest (herein called EASI) in an ambient medium; more particularly, this invention relates to an improved membrane-enclosed amperometric cell for use in such methods.
(b) Description of the Prior Art
Electrochemical cells for quantitative electrochemical analysis are well known in the art and generally include a working or sensing electrode having a generally flat and frequently circular sensing area covered by a thin film of a liquid electrolyte that extends into an electrolyte reservoir and is in electrolytic contact with a counter electrode; a flexible polymer membrane that is substantially impermeable to the electrolyte but permeable to the EASI (generally called "semipermeable") extends in a substantially conforming manner over the sensing area of the working electrode as well as the electrolyte film thereon. Such a "membrane-enclosed amperometric cell" will be called a MEAC hereinafter.
For amperometric analytical operation, the working electrode of such a MEAC is polarized by a constant DC voltage to furnish a current whose steady state magnitude is proportional to the activity of the EASI. Such MEAC's, their operation, and their use for determination purposes are discussed in the following illustrative U.S. Pat. Nos. 2,913,386, 4,096,047, 4,325,797 and in British Specification No. 2,013,895.
Structural and operational data of such prior art cells used for oxygen sensing are to be found in the literature, particularly in the Monography by Hitchman, Michael L., "Measurement of Dissolved Oxygen", John Wiley & Sons, Inc. and Orbisphere Laboratories, 1978.
While elemental (molecular or O.sub.2) oxygen is a preferred EASI, others are of interest here as well and include elements or compounds that are more easily oxidized or reduced in the cell than the electrolyte (solvent and solvate); elemental hydrogen is another preferred EASI if measurement is made in line with the method disclosed in the above cited U.S. Ser. No. 493,316. The ambient medium may be gaseous or liquid and will generally contain the EASI in an essentially pure form or in an admixed or in a dissolved state, the EASI concentration varying between 100% and parts per million(ppm) or parts per billion (10.sup.-9).
Depending upon whether the EASI are of the electroreducible type, such as oxygen, or of the electrooxidizable type, such as hydrogen, the sensing or working electrode of the MEAC will be the cathode or the anode, respectively, while the counter electrode will be the complementing electrode and suitable insulator means, i.e. non-metallic, inorganic or organic solids, are provided between the electrodes so that any current which is permitted to pass from the sensing electrode to the counter electrode is a ionic current in the electrolyte arising from electrochemical phenomena at the electrolyte-exposed electrodes.
For operation of a MEAC, the semipermeable membrane will be secured on the cell after the electrolyte-receiving portion including the sensing area of the working electrode is provided with the electrolyte which will be exchanged with the membrane for maintenance.
That portion of the EASI-exposed surface of the membrane in operative position and separating the ambient medium from the electrolyte is also called the "sensing face" of the MEAC; frequently, the sensing face will be a transverse and generally circular front face of an elongated tubular housing or jacket onto which the membrane is fastened. Normally, the housing or jacket material will be substantially impermeable to the EASI and the electrolyte-backed membrane portion should be the only part of the MEAC where the EASI can get into the electrolyte film or layer on top of the sensing area of the working electrode.
It will be appreciated that the electrolyte-covered and membrane-covered "sensing area" of the working electrode will generally be an essential and frequently central but not necessarily predominant portion of the membrane-covered "sensing face" of the MEAC.
The particular importance of the electrolyte film on top of the sensing area of the working electrode and the accessibility of this film to EASI will be explained in more detail below.
When measuring the concentration of an EASI in a fluid medium that contacts the sensing face of the MEAC, the desired current contribution normally originates from diffusion of the electroactive species directly through the membrane onto the sensing area of the working electrode and the corresponding electrochemical reaction of the EASI on the working electrode. In practice, however, additional and undesirable current contributions, i.e. those unrelated to the concentration of the electroactive species of interest in the medium, are observed and limit both accuracy and sensitivity of the measuring system, aside from causing problems of stabilization of the transient signal, stability of the steady state signal, undesired noise signals, and prolonged response time.
One specific type of undesirable current contributions is that caused by electrolyte penetration into the interface between the working electrode and the adjacent insulator portion as set forth by Applicant in the above cited U.S. Pat. No. 4,096,047 (incorporated herein by reference) and disclosing means to avoid such penetration by pressure sealing instead of conventional cementing.
Further research has shown that a predominant portion of the undesirable current contributions is due to diffusion and leakage effects. For example and with reference to the oxygen or hydrogen measurement as typical examples, the EASI may penetrate into the electrolyte remote from the sensing area of the electrolyte, e.g. via the membrane/housing junction, a housing/electrode junction, an electrode/insulator junction, etc. These EASI constitute an "impurity" in the system and tend to diffuse "laterally" from the electrolyte space or reservoir into the electrolyte film on top of the sensing area of the working electrode where they will react and cause a current not related to the concentration of the EASI in the ambient medium that is in contact with the membrane surface directly adjacent the sensing area of the electrode. EASI diffusion or leakage into electrolyte portions other than the film on the sensing area of the working electrode and subsequent lateral diffusion into said film would thus be the primary cause of these undesired current contributions. However, when attempting, for example, a sensitivity of the amperometric oxygen detection from the parts per million [10.sup.-6 ] (ppm) range into the parts per billion [10.sup.-9 ] (ppb) range it is apparent that there is a limit to materials and structures that would be required for complete elimination of EASI leakage.
To the best knowledge of Applicant, the most effective prior art method for avoiding undesired current contribution caused by EASI leakage or undesired diffusion is to provide a third electrode commonly called a "guard" as disclosed, for example, in the above cited British Specification 2,013,895 and acting as an electric barrier against lateral diffusion of EASI onto the working electrode as explained below.
However, when working with advanced hydrogen determination methods as disclosed in the above cited U.S. Ser. No. 493,316 (incorporated herein by reference) it was found that even a guard electrode cannot eliminate all undesired current contributions in that--for example--an organic insulator between the working electrode and the guard may cause a residual current after hydrogen exposure of the MEAC because of the high solubility of hydrogen in most organic polymers so that the hydrogen dissolved or otherwise retained by this or another component of the MEAC will continue, for some time, to cause an undesired current contribution.